This invention relates to the acoustic testing of armatures of dynamoelectric machines. More particularly, this invention relates to the ultrasonic testing of fused joints between the tangs and commutator bars of such an armature.
The armature of a dynamoelectric machine--i.e., of an electric motor or generator--has at least one wire coil wound thereon. Electrical power is conducted to the coils through a commutator which circumscribes the armature shaft and contacts electrical brushes on the stationary part of the dynamoelectric machine. The commutator is made of an even number of commutator "bars" spaced around the shaft. Each end of each coil terminates at a commutator bar.
A commutator bar is generally a substantially rectangular piece of copper. If the number of commutator bars in the commutator is small, each bar may have a slight curvature so that the commutator as a whole is substantially round. Where the number of commutator bars is large, it is not necessary for the bars to be curved.
A commonly employed method for terminating the coil end leads to the commutator bars is to provide on each commutator bar a small finger-like extension known as a "tang." The tang is bent at an angle to form a hook around which the coil lead is wrapped. The tang is then bent over parallel to and touching the commutator bar, so that the lead is trapped. The surfaces of the tang and the commutator bar that are in contact are then joined to prevent the tang from unfolding and freeing the lead.
A common method of joining the tang surface to the commutator bar surface is called "fusing" or "hot-staking." In this method, the surfaces are joined by applying pressure on the tang with a probe, forcing the tang against the commutator bar, and then heating the joint by either heating the probe or passing an electric current from the probe through the joint. The combination of heat and pressure causes projections and depressions on one surface to interengage with depressions and projections on the other surface to form a frictional joint; no significant melting of either surface occurs. In addition, an insulating coating on the wire trapped in the joint vaporizes, allowing the wire to make electrical contact with the commutator bar.
Because of the nature of fusing, fused joints will almost always contain gaps. Usually, these gaps are small "microgaps" dispersed throughout the joint, but larger gaps could also be present. The number, size and distribution of gaps determines the quality of the fused joint. Joint quality can be measured, for example, by the percentage of the area of the joint that is occupied by gaps; the higher the percentage, the worse the joint.
In practice, however, it has not heretofore been possible to directly measure the gap percentage. Instead, a test known as the "click test" has commonly been used. In the click test, a mechanical probe is manually inserted into the joint by an operator who applies force to break the joint open. The breaking of the joint produces an audible click sound, and the operator is trained to distinguish the sound of a good joint from that of a bad joint. The test is thus highly subjective, leading to variations in quality depending on the operator's perceptions. In addition, the test is destructive, and tested armatures must be discarded. Statistical assumptions must be made with respect to untested armatures, which are the only ones that can be used.
In a refined version of the click test, the probe is equipped with a force gauge, and the force needed to break open the joint is measured. When the force exceeds a threshold, a good joint is indicated. Although this test is more objective, it is still destructive and must rely on statistical assumptions with respect to the untested armatures. Even if the application of force were stopped at some level deemed appropriate (assuming the joint has not opened at a lower level of force), thus saving the armature, such a test would be too slow to use on all armatures. Statistical assumptions would still be required.
Thus, it would be desirable to be able to provide an objective and non-destructive test of the fused tang/commutator bar joints in an armature.
It would also be desirable to be able to provide such a test that could be used, if desired, on every armature being manufactured on an armature manufacturing line.